Multiple sclerosis (MS) is an immune-mediated central nervous system (CNS) disease characterized by perivascular CD4 + T cell and mononuclear cell infiltration with subsequent primary demyelination of axonal tracks leading to progressive paralysis. MS is generally considered to be an autoimmune disease involving autoreactive T cell responses to MBP, PLP, and/or myelin-oligodendrocyte glycoprotein (MOG), however a clear cut cause-effect relationship between myelin reactivity and disease pathology has yet to be demonstrated. Although MS is generally considered to involve an autoimmune pathology, little is known regarding its etiology and there are limited therapeutic strategies available to specifically inhibit and prevent ongoing disease. We propose to investigate intrinsic regulatory mechanisms that influence both the susceptibility to and progression of MS using two mouse models of experimental autoimmune encephalomyelitis (EAE) that either follow a relapsingremitting (R-EAE) or chronic (C-EAE) disease course. CD4+CD25 + regulatory T cells (TR) are potent inhibitors of CD4 + T cell responses that are activated in an antigen-specific manner via TCR cross-linking and have been shown to mediate protection against the initiation of several spontaneous autoimmune disorders, such as diabetes and autoimmune gastritis. Our preliminary data clearly demonstrate that supplementation of TR significantly protects against the initiation of EAE by inhibiting the proliferation and effector function of both naive and previously-activated encephalitogenic CD4 + T cells, while depletion or inactivation of these cells leads to exacerbated disease symptoms. We have also shown that TR-mediated disease inhibition is associated with an increased frequency of myelin peptide-specific protective Th2 cells within the peripheral lymphoid organs and significantly decreased numbers of mononuclear cells infiltrating the CNS. The experiments in this proposal will test the hypothesis that CD4+CD25 + TR play a critical role in conferring resistance to EAE susceptibility and in regulating disease progression by inducing antigen-specific unresponsiveness to disease-relevant self-myelin peptides. The proposed experiments will elucidate the cellular and molecular mechanisms by which TR inhibit the functional activity of encephalitogenic effector Thl cells in acute and relapsing EAE, thus blocking the initiation/progression of clinical demyelinating disease. Three specific aims are proposed: AIM 1 will determine the role of CD4+CD25 + TR in modulating the initiation, progression, and recovery phases of R-EAE and C-EAE. AIM 2 will determine the contribution of CD4+CD25 + TR to age-, gender-, and strain-associated susceptibility to EAE. AIM 3 will elucidate the mechanisms by which CD4+CD25 + TR influence encephalitogenic T cell activation, expansion, trafficking, and/or effector function. These studies should further our understanding of the role of TR in regulating both the induction and progression of CNS autoimmunity and provide vital information relative to optimizing strategies designed to enhance intrinsic regulatory mechanisms for the prevention and treatment of human autoimmune diseases.